The present invention concerns a method and a device of transmitting uncompressed video streams. In particular, the method concerns the adaptation of a wireless transmission to bandwidth reduction and packet loss.
New recent multimedia applications, such as high-definition audio/video streaming, require wireless transmission of uncompressed video at high data rate of about several Gbps (Gigabits per second), with low latency.
As an example, high definition (HD) video generally refers to a sequence of 60 Hz video frames with 1920 vertical lines and 1080 horizontal lines corresponding to 1920×1080 pixels of 24 bits. A video wireless communication system that supports such HD video, referred to as uncompressed HD 1080p video, requires a channel bandwidth of about 3 Gbps to support video data only. Such rate is not achievable in current 802.11 wireless communication systems using the 2.4 GHz and 5 GHz radio bands. To overcome the limits of these radio bands, higher frequencies, for instance the 57-66 GHz millimeter-wave unlicensed spectrum, referred to as 60 GHz millimeter wave technology, are used.
60 GHz-based communication systems are widely studied (e.g. IEEE 802.11 Task Group; IEEE 802.15.3c standard; Wireless HD; WiGiG; etc.) and the research community proposes several solutions and methods to transport the audio and video applications with a desired quality of service (QoS). In wireless communication systems, before a video stream is transmitted, connection setup and channel bandwidth reservation are typically conducted. Ideally, sufficient channel bandwidth can be allocated and the video stream can be transmitted smoothly after stream set-up control.
However, it may happen that the channel bandwidth is not sufficient. For instance, the video stream may not receive sufficient channel bandwidth due to other concurrent transmissions on the same channel or due to the high sensitivity of the 60 GHz millimeter wave technology to perturbations, such as shadowing or interference, or fading phenomena making the quality of the wireless channel dynamically change over time. For instance, 60 GHz wireless channels with beam-formed transmissions may be affected even by a person moving in the neighbourhood of the emitter or receiver which appears as an unexpected obstacle on a transmission path. In these cases, the degradation of the communication channels requires, for example, changing the channel coding, for instance by increasing the redundancy data, or/and the type of modulation of video data. Some other unexpected perturbations may also happen as synchronization problem or PHY preamble loss. Such problems result in the initial allocated channel bandwidth being no longer sufficient for the transmission. Some packets have to be discarded. But a packet contains a portion of the video and typically video components of adjacent pixels. Consequently, when a packet is discarded, an area of the video is not displayed and provokes a poor video rendering for the users.
In order to cope with these unexpected perturbations, a typical way is to introduce an error-control method at MAC level such as an Automatic Repeat reQuest (ARQ) scheme in order to optimize the reliability of the transmission. In a typical ARQ method some acknowledge messages are exchanged between the receiver and the emitter to enable the retransmission of these erroneous packets. For this purpose, the sender introduces a Cyclic Redundancy Check (CRC) in each sent packet. The receiver uses this CRC to determine whether the packet is erroneous or not and indicate the output in a message referred to as acknowledge message or ack. The acknowledge message is sent from the receiver to the sender. According to the value of the ack, the sender retransmits the erroneous packets. There are several known strategies to manage the error detection, the retransmission (selective repeat, go back n, . . . ) and the acknowledgements (cumulative, selective, . . . ). A main feature often used for the high rate transmission is the block acknowledgement: instead of transmitting an individual acknowledge message for each packet, multiple packets can be acknowledged together using a single acknowledge message. It allows reducing strongly the overhead introduced by the mechanism and to improve the latency.
Typically, an ARQ scheme is able to recover up to a fixed packet error rate, referred to as the capacity of the ARQ scheme, which depends on the available bandwidth dedicated to retransmission in order to preserve the data throughput. Typically, in 802.15.3c or wireless HD, an ARQ scheme is able to recover a Packet Error Rate (PER) between 0 and 2%. The available bandwidth for retransmission data is fixed at the initialization of the system and depends on several configuration parameters such as acknowledge scheme, aggregation scheme and so on.
Unfortunately, when the PER exceeds the capacity of the ARQ scheme, some packets to be retransmitted have to be discarded arbitrary. Consequently, an area of the video is not displayed which provokes a poor video rendering for the users.